Process gases such as those gases used for semiconductor processes (e.g., arsine (AsH3), phosphine (PH3) and ammonia (NH3)) typically must be purified prior to use in semiconductor processes in order to remove impurities (e.g., moisture, CO2 and SO2) to acceptable levels. For example, ammonia is used as a source gas in the chemical vapor deposition (CVD) of nitride films during the fabrication of semiconductor chips. Typical nitrides are silicon nitride, made for example by the reaction of silane and ammonia and titanium nitride, made for example by the reaction of titanium tetrachloride and ammonia. The presence of one to three thousand parts per billion (ppb) levels of moisture can result in a decrease of the performance properties of the nitride layer. Gallium nitride CVD technology has been shown to require even lower levels of moisture in the source of ammonia than silicon and titanium nitride technology.
Purification of such gases either prior to use in semiconductor applications or in purification after use and before re-use currently includes passing the process gas through one or more beds formed of an adsorbent to remove such target impurities.
U.S. Pat. No. 6,576,138 B2 to Sateria et al. discloses a method for producing purified semiconductor gases from its impure form using adsorption and evaporation techniques.
U.S. Pat. No. 6,749,819 B2 relates to a process for purifying ammonia. The process relates to contacting crude ammonia with catalyst to remove oxygen and/or carbon dioxide that are present in the ammonia as impurities. The reference also relates to contacting crude ammonia with a catalyst that contains manganese oxide as an ingredient and then contacting the ammonia with a synthetic zeolite to remove at least one of carbon dioxide, oxygen and moisture from the ammonia.
Adsorption processes can be sensitive to temperature. Generally, higher operating and/or ambient temperatures will reduce adsorption efficiency. In addition, gas purification systems are often located in geographical areas where the ambient temperature is not controlled or monitored, and/or can fluctuate significantly based on prevailing weather and other environmental conditions. Consequently, there can be significant variability in the temperature of the adsorbent bed. The adsorption process itself moreover results in an increase in the temperature of the adsorbent bed, especially in cases where the moisture content of the feed material is high. These factors can cause the quality of the material being purified to fluctuate on the basis of the operating temperature of the adsorbent bed and/or surrounding conditions.
Another factor to be considered with regard to the purification of such gases includes the time required for activation of the adsorbent bed(s). The activation process involves heating the adsorbent bed(s) to elevated temperatures (for example, 100° C. to 400° C.) while flowing a dry purge gas through the bed(s). Because many adsorbents used in purification systems for semiconductor gases are of low thermal conductivity, it can be difficult to control the temperature of the adsorbent bed not only during purification operations, but also during the activation phase of the adsorbent bed.
Previous attempts to address heat transfer issues have involved several approaches. For example, the use of monoliths of carbon with higher thermal conductivity than granular adsorbents as the adsorbent media in the bed to improve heat transfer is proposed in Menard, et al., “Activated Carbon Monolith of High Thermal Conductivity for Adsorption Processes Improvement Part A: Adsorption Step”, Chemical Engineering and Processes, 44 (2005), 1029-1038.
A method for heat transfer in the bed in which a finned inner-tube is enclosed in a larger tube and the adsorbent material is contained in the annulus thus formed between the two tubes is contemplated in Bonjour, et al., “Temperature Swing Adsorption Process with Indirect Cooling and Heating”, Ind. Eng. Chem. Res. 2002, 41, pages 5802-5811.
Some general means of removing moisture from fluids is discussed in Weiner, “Dynamic Fluid Drying”, Chemical Engineering, Sep. 16, 1974, pages, 92-101.
U.S. Pat. No. 4,165,972 to Iles et al. proposes heating and cooling of the sorbent beds by flowing coolant through the beds.
U.S. Pat. No. 5,169,413 to Leavitt relates to a pressure swing adsorption (PSA) system which includes means for controlled retention of internally generated, self-refrigeration such that the average temperature of the bed is to be reduced.
U.S. Pat. No. 5,268,022 to Garret et al. proposes a method for heat transfer within a bed that involves having at least one heat conductive member containing a liquid medium arranged within the bed such that in operation, heat is said to be conducted by convection through the liquid from a region of maximum temperature at or near the bottom of the bed to a region of minimum temperature at or near the top of the bed.
There remains a need in the art of gas purification for improved temperature control of adsorbent beds such that the purified gas produced from such processes contain reduced impurity levels and/or exhibit less variability in impurity concentration. There also remains a need to reduce the time required to activate adsorbent beds prior to use and/or reuse in such purification processes.